a. Field of Invention
The invention relates to multi-beam antenna systems and, more particularly, to a focal plane array (FPA) antenna system that includes a multi-stage interference cancelling adaptive digital filter that allows reception of multiple telemetry streams simultaneously.
b. Background of the Invention
Improving the efficiency of aeronautical telemetry systems often entails dealing with an interference problem known as multipath. Multipath is created by telemetry reflections from various surface/objects, and it usually includes a strong “ground bounce” with complex amplitude and frequency characteristics. As the data rates used for aeronautical telemetry increase, multipath interference is becoming increasingly frequency selective and significantly impairs reception. For example, current United States Air Force (USAF) flight test telemetry ranges commonly suffer from time-varying frequency-selective multipath due to ground reflections of radio-frequency (RF) telemetry signals.
For this reason various smart antenna systems have been developed to reduce multipath. Generally, these smart systems employ multiple antennas in a phased array, followed by some form of diversity processing to combine the multiple signals received into a more accurate whole. These smart systems employ two different combining schemes: 1) diversity combining; and 2) adaptive combining.
The diversity combining scheme exploits the spatial diversity among multiple antenna signals received. Diversity combining is illustrated in U.S. patent application No. 20020067311 by Wildey et al. published Jun. 6, 2002, which shows a phased array antenna with a plurality of antenna elements. The beacon signal is passed to a beacon signal processor which determines the phase differences between the signals received at different antenna elements. The phase differences provides a measure of the physical displacement of the antenna elements from their nominal relative positions, due to distortion of the antenna structure resulting from, for example, gravitational forces. The antenna system can further include means for generating a phase correction corresponding to the phase difference to alter the communication signal phasing and so compensate for distortion of the array structure. This essentially provides a self-phasing phased array in which the radiation pattern automatically adjusts to compensate tor displacement of the antenna elements from their nominal positions relative to one another.
U.S. Pat. No. 5,680,419 to Bottomley (Ericsson) also suggests the elimination of the deleterious effects of fading, time dispersion and interference by using interference rejection and diversity combining.
In contrast, the adaptive combining scheme adjusts the antenna weights dynamically to enhance the desired signal while suppressing interference signals. Most adaptive combining techniques employed today use complex weights on each of the antenna receivers to reject interferences, but in the presence of multiple paths of propagation. However, they tend to “aim” in the direction of one of the paths, thus losing the energy associated with the other paths.
Spatio-temporal equalization (S/T Equalization), which utilizes both spatial and temporal information of received signals, has been drawing much attention as a technique to achieve better performance in antenna systems. By manipulating the specific phase and amplitude relationship of the signals received at the phased antenna array, it is possible to correct various distortions such as reflector surface aberrations in satellite and other communications. This is achieved by controlling signal power division ratios (spatial equalization) and the phase shift (temporal equalization) in the path between the signal source and each antenna element. Spatial diversity equalizers use various algorithms. For example, U.S. Pat. No. 6,006,110 to G. G. Raleigh describes a time-varying vector channel equalization approach for adaptive spatial equalization. Spatio-temporal equalization (S/T-Equalization) can achieve significant enhancement in signal transmission performances over broadband mobile communication channels.
Traditional S/T-Equalization methods employ an adaptive antenna array which has temporal filter at each antenna element (a broadband beam former) or a complex decision feedback equalization scheme which requires a lengthy training sequence, such as a maximum likelihood sequence estimation (MLSE) filter. Though S/T-Equalization methods can achieve good performance, they are currently limited to the aperture plane of a conventional phased array antenna system.
Focal plane array antenna systems differ significantly from phased antenna arrays. Rather than using multiple antennas, a focal plane array feed entails arranging multiple antenna elements within the focal plane of a single parabolic dish antenna in a specific geometry. For example, U.S. Pat. No. 6,320,553 patent to Ergene (Harris) shows a main parabolic reflector and an ellipsoidal subreflector, with a transversely positioned feed and an axial feed located in the focal region of the main reflector. In this case the patent notes that mutual blockage can occur between several different feeds in the same antenna configuration, and the two feeds are herein designed to receive and transmit signals in different frequency bands (e.g. C, KU and X-bands) from a single antenna dish system without blockage.
Though S/T-Equalization methods have used in to the aperture plane of a conventional phased array with multiple antennas, they have not been widely applied to the local plane of a parabolic dish antenna. Nevertheless, S/T-Equalization in focal plane array antennas can endow receivers with the immunity against co-channel interference (CCI) and inter-symbol interference (ISI), aiming at allowing all users to use the same frequency- and time-slots without spreading their signals in the frequency domain. See, for example, M. Rice and E. Satorius, Equalization Techniques For Multipath Mitigation In Aeronautical Telemetry, MILCOM 2004 Proceedings, volume 1, pages 65-70 (Oct-Nov 2004), which proposed a single channel equalizer to improve the performance of telemetry reception systems. This reference discloses a single-channel equalizer with spatial but not temporal processing.
The present inventors herein demonstrate that it is possible to extend the single-channel equalization scheme in a focal plane array to multiple spatial channels in order to fully exploit spatial diversity, thereby achieving further reductions in interference, resulting in even more reliable telemetry reception. The invention described herein applies spatio-temporal equalization techniques, not to the aperture plane of a conventional phased array, but instead to the focal plane of a single parabolic dish antenna, it is possible to improve its performance under multipath conditions while maintaining high precision pointing capability. A focal-plane tapped delay-line approach is herein proposed that is fundamentally different from the phased-array approaches described above, since focal-plane processing of multipath signals and other interfering sources can be separated out without complicated signal processing (under favorable conditions). This is an added bonus of using focal-plane arrays instead of conventional phased-array antennas, when appropriate. Moreover, this approach makes it possible to provide a single-antenna system capable or receiving telemetry from two or more sources within the field-of-view, even in the presence of multipath. This capability promises significant value whenever it is desireable for one antenna to receive more than one stream simultaneously under many conditions, such as in aeronautical activities where existing parabolic ground antennas could receive telemetry from formation flyers within the fields-of-view.